Mass spectrometers are instruments used to analyze the mass and abundance of various chemical components in a sample. Mass spectrometers work by ionizing the molecules of a chemical sample, separating the resulting ions according to their mass-charge ratios (m/z), and then detecting the abundance of ions at each m/z. The resulting spectrum can be interpreted to reveal the relative amount of each chemical component in the sample based on the abundance of the mass fragments of these components.
Various mass spectrometers generate ions from the sample utilizing various methods, for example, electrospray ionization, atmospheric pressure chemical ionization, matrix-assisted laser desorption/ionization, and inductively coupled plasmas. In some situations, the ion source that generates the ions is located external to a mass analyzer. The ions are guided from the ion source into a mass analyzer, where the ions are separated based on mass. The ions then arrive at a detector that detects charge and/or current. Information based on the detected charge and/or current is then used to determine the quantity of ions of various masses.
One type of mass analyzer used for mass spectrometry is called a quadrupole ion trap. Quadrupole ion traps take several forms, including three-dimensional ion traps, linear ion traps, and cylindrical ion traps. The operation in all cases, however, remains essentially the same. DC and time-varying radio frequency (RF) electric signals are applied to the electrodes to create electric fields within the ion trap. These fields trap ions in a “cloud” within the central volume of the ion trap. By manipulating the amplitude and/or frequency of the electric fields, ions are selectively scanned out by being ejected from the ion trap in accordance with their m/z. A detector records the number of ejected ions at each m/z as they arrive.
Ion traps are optimized for a combination of speed, sensitivity, and resolution depending on the particular application. For a given instrument, an improvement in one category is usually made at the expense of another. For example, sensitivity can generally be increased by using a slower scan, and in the reverse, a scan can be performed faster at the expense of sensitivity. Similarly, sensitivity—especially to less abundant components of a sample—can be increased by trapping and scanning a larger total number of ions in a single scan. However, as the quantity of ions in the trap increases, the coulombic forces and collisions between the like-charged ions in the ion cloud increases, resulting in space charge effects. Mass spectrometers achieve resolution by ejecting all ions of the same m/z at close to the exact same moment. However, when space charge effects become significant, ions are ejected from the trap at different times. The result is broadening of spectral peaks and loss of resolution. Also, detectors used in mass spectrometers typically have a limited dynamic range, the difference between the lowest and highest concentration that can be detected. Concentrations lower than the lower bound are undetectable due to, for example, noise; and concentrations above the upper bound may saturate the detector. Additionally, mass analyzers may trap ions preferentially based on their mass, thus for a sample with a range of masses, larger ions may not be trapped as efficiently as lower masses.
There is a need for systems and methods for expanding the range of concentrations detectable by mass spectrometers. The present disclosure is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.